While working on a seperate issue, I found that my crankcase seems to be under an abnormally high vacuum from the pcv. With the engine idling hot, if I pull the dipstick the idle raises and there's a lot of vacuum at the tube end. If I open the oil fill cap the enine stalls. Have not had this happen on other MAF engines. Searching around the web, this seems to indicate the pcv valve is stuck open. My questiom: am I correct that the valve should reduce vacuum? Service manual has next to no info on the pcv system. Also, is it normal for the engine to stall when opening the oil fill cap on these engines?

Thanks,
Michael
​​​

 

Well, answered my own question. Tested the pcv valve per directions on this forum, doesn't rattle and doesn't block flow in either direction. Ordered a new one from Jag. Will post back about the stalling after I get the new one.

 

The AJ v8 is a sealed crankcase system. You should never run the vehicle with the dipstick removed, or the oil filler cap off or loose. Unmetered air is drawn into the system, and not seen by the MAF which will cause you heartache you do not want.

 

Hello (Sir) Michael,
My PCV Valve doesn't rattle and doesn't block flow in either direction - I assume from your post that I need a new one?

However, if I block the pipe that fits into the Cam Cover and suck on the pipe that goes to the inlet, I can hear/feel the diaphragm closing against the spring and as it does so air is drawn in through a tiny square hole let into the underside of the outer rim of the valve body (presumably to allow air in on the non-functioning side of the diaphragm, so that it can be drawn against the spring). If I remove my mouth, the spring returns the diaphragm and expels air through the tiny hole as it does so.

However, if I block the pipe to the inlet and suck on the pipe to the Cam Cover, the valve closes and remains closed when I remove my thumb but continue to maintain the sucking 'vacuum'?

If I block neither pipe I can blow and suck through both pipes with no effect whatsoever?

Is this valve functioning okay - What do you think?

Regards,

Ray

 

Not sure how the PCV is supposed to function. Will have a new one here Thursday and will report back.

 

Hello Michael, Thank you - I am time-pressed for answers because of the need to order parts from the UK/USA and then have them delivered to the Middle East, so consequently I have posted this topic as a new post.

A new PCV Valve will cost me £30 for the valve and a further £25 delivery!

I will be very interested to hear how your new PCV valve behaves if you suck, block & blow though it in all directions!

Regards,

Ray

 

Michael, I can give you an opinion regarding how a PCV Valve functions. It introduces oil-laden vapours from the crankcase into the inlet manifold so that my plugs become coated and my oil and my engine gets degraded with carbon sludge; how burning oil in your cylinders creates lower emissions escapes me!

 

Hi I had a spare one in the garage so i put it on the car this last weekend, before I put it on I sucked on it and blew on it and there was no resistance either direction, the old one is the same, also there is no rattle when shaken, not like the metal ones in some cars, I dont know but i think if the engine is in good shape only fumes pass through this part, if the engine is old and worn then oil and fumes would get passed through to the throttle body, anyhow my car is running good as it was before the change, and performance and gas consumption is good, FYI I am retired and get bored easy, always tinkering with the car
regards
Dave
2007 xj8

 

Hello Dave,

Thank you for your reply. FYI I am old and retired and I have been tinkering with old cars for years; probably why I have a low threshold for more modern cars designed for low emissions instead of optimum performance and economy!

 

Exactly why I use an oil catch can!

 

Thank you jackra_1 , you can probably guess that I am going to ask you what is an oil catch can and if it is such a good thing, how can I get one, please?

Regards,

Ray

 

Hello Michael, One of the things that's puzzling me about this valve is that I have seen one of its functions described as a 'Check Valve'. From what I understand (admittedly very little) if you have a backfire (misfire, they appear to be called nowadays) in your Inlet Manifold, this valve stops the pressure from expanding into your Crankcase? Accordingly, I would expect this valve to close abruptly if you blow hard into the top of it (i.e. from the inlet)? But it doesn't??

Or does there have to be an opposing pressure from the Crankcase before this 'Check Valve' function initiates, being as allowing pressure out of the Crankcase is the valves primary purpose?

I have rechecked it again and there does not appear to be any scenario wherein the valves slams shut when you blow back through it?

 

That is usually correct for a PCV valve. You don't want a backfire into the crankcase. My new PCV has not arrived yet, though it was supposed to be here yesterday

I'm really curious to know how a new valve is supposed to work. I'm wondering if that old thread is correct or not (https://www.jaguarforums.com/forum/x...-valve-134269/). That method is the way you test an old style PCV like this:



I'll be a bit upset if ordered a new PCV based on poor information in an old thread. Not sure ours should actually rattle when shaken. Will post back as soon as I know more!

-Michael

 

The design for crankcase ventilation for the AJ8 is not as unique, though not as common as other systems. The reason it is a closed system is twofold, one to contain hydrocarbons that are measured for emissions testing, and a closed system who's input is measured as part of the fuel trims by the Denso electronics.

The valve itself is a rubber diaphragm loaded with a spring, which when intake vacuum is applied, overcomes the pressure of the spring and allows for a metered amount of crankcase fumes to be drawn into the intake and burned during the normal combustion process. When low vacuum is present, a much larger amount of air passes under high load. And since the crankcase does have oil mist in it, you will see that oil being drawn into the intake track. This is normal operation. What isn't normal is a ruptured diaphragm that allow high evacuation at all times.

 

Thank you Box,

So (please correct me if I have this wrong) what you are saying is that the default position for this valve is closed? Remains closed until the inlet vacuum exceeds the strength of the spring holding it closed? Accordingly, a valve under no applied vacuum which remains open is shot?

I ask because my valve only closes if I block one end with my thumb and suck on the other end. If the end that I suck on is the end going to the inlet, when I remove my thumb from the end which fits in the Cam Cover, the vacuum is lost; however, if the end I am sucking on is the end that fits in the Cam Cover, the vacuum remains (and presumably the valve remains shut) when I remove my thumb from the end that goes to the inlet, providing I continue to suck on it?

Regards,
Ray

 

No Ray, that is exactly opposite of it's operation. It is wide open at no vacuum.

 

Hello David. Thank you. But I do not understand. Box is saying that the vacuum applied by the inlet sucks the diaphragm against its spring to open the valve? If so, how can it possibly be wide open at no vacuum, unless it has failed?

David, I am simply trying to determine whether my valve is functional or needs replacing? What I am saying is that my valve 'at rest' is open and if I suck on my inlet the valve remains open and does not close until I block the pipe coming from the Cam Cover; given this scenario, the valve will always be open unless the pipe from my Cam Cover becomes blocked and accordingly, it performs no useful function that I can see or understand?
Now, if my valve was closed when at rest and opened against its spring when I applied a vacuum at the part leading to the inlet, then that would make sense to me?

Regards,
Ray

 

I think it's just a matter of phrasing. The way a PCV is supposed to work is to restrict the vacuum so at idle and closed throttle, so that the crankcase isn't exposed to full manifold vacuum and it will regulate the flow of air through the PCV system. But when you're accelerating or at WOT, the manifold is under little/no vacuum so the PCV valve needs to be wide open to allow flow of vapors.

This is the very issue I'm having with mine. It's letting manifold vacuum be applied to the crankcase without restriction at idle. The MAF doesn't know any different, because the air is all being metered, but the PCV is allowing all that extra air to bypass the throttle plate. Hence when I open my fill cap it's experiencing a massive manifold leak. There are other PCV designs where the "valve" is just a fixed orifice.

My new valve was delayed in shipping but may be here today or at latest tomorrow.

 

The condition you describe is normal operation. The PCV system is a metered vacuum leak as part of the fuel trims. Never try to operate without the oil fill cap installed, and both the cap, it's stand, as well as the dipstick need to provide positive sealing. Both of you are hunting a problem that isn't a problem, but by design.

 

I understand that the PCV is a sealed system. But on every other MAF vehicle I've tried, I can open the oil fill cap with the engine idling and the engine doesn't stall. It might idle rougher, but not die immediately.

Received the new OE PCV valve just this evening. Sadly the old one does seem to be operating correctly. There is no resistance to flow in either direction, but if you cover one port and apply vacuum the internal diaphragm closes.

So the link referenced in post #13 has the incorrect test procedure for the PCV in this engine. There is no rattle and there is no one-way check valve. It just controls flow based on the pressure differential.